Conventionally, a so-called neutral control is performed so as to control an automatic transmission to be neutral when a vehicle is stopped during a shift lever positioned at a D-range, thereby capable of reducing slip loss of a fluid in a torque converter. However, a vehicle driving force is substantially disappeared when the automatic transmission has been controlled to be neutral so that the vehicle may move in a rearward direction on an uphill slope. In order to prevent the vehicle from moving in the rearward direction during the neutral control, a hill hold control has been highly valued, which is performed by use of an output shaft reverse rotation restrict mechanism housed in the automatic transmission.
The forgoing type of control is disclosed in a Japanese Patent Publication published as Toku-Kou-Hei 5(1993)-86514. An automatic transmission disclosed in the aforementioned publication is provided with a forward clutch, which connects a torque converter and the transmission, a one-way clutch, which cooperates with the forward clutch and establishes a forward shift stage when being engaged, and a shift brake, which cooperates with the one-way clutch and prevents a vehicle from moving in a rearward direction when being engaged. While the vehicle has been temporarily stopped with a vehicle brake-operating member being operated, an engaging pressure applied to the forward clutch is set to be substantially lower than an engaging pressure actually applied to the forward clutch during the forward shift stage being established in the automatic transmission and the shift brake is also engaged. Corresponding to release of the vehicle brake operating member from the operated condition, the engaging pressure, which has been applied to the forward clutch under the above-described circumstances, is increased up to a level of the engaging pressure actually applied to the forward clutch under the forward shift stage. The shift brake is released from the engaged condition in a period of time after the initial increase of the engaging pressure applied to the forward clutch. The period of time is prescribed in accordance with the increase of the engaging pressure applied to the forward clutch.
According to the automatic transmission described above, the shift brake is released from the engaged condition in the predetermined period of time. Therefore, the shift brake may be released from the engaged condition before the forward clutch is engaged due to dimensional fluctuation of the forward clutch or the shift brake or due to fluctuation of the controlled oil pressure, thereby the hill hold condition may be released before the neutral shift stage is shifted to the forward shift stage. In this case, the one-way clutch may not be able to effectively prevent the vehicle from moving in the rearward direction and the vehicle may move in the rearward direction so that a driver may obtain an uncomfortable start feeling.
Accordingly, the present invention therefore seeks to provide an automatic transmission, in which neutral and hill hold controls are performed during a vehicle being stopped so as to achieve a preferable start feeling.